1. Field of the Invention
The invention relates to a frequency correction method intended to correct the frequency of a local carrier in a receiver of a data transmission system in which the data are transmitted by carrier modulation and a receiver synchronizing word is transmitted before the data, this receiver comprising means for demodulating a received signal with the aid of signals in phase and in quadrature with a local carrier, means for sampling the inphase and quadrature demodulated signals with a sampling frequency equal to the modulation rate, each new pair of in-phase and quadrature samples thus formed being written into a received-signal memory, the receiver further comprising a synchronizing-word memory containing the in-phase and quadrature samples of a local synchronizing word which is identical to the transmitted synchronizing word.
The invention also relates to a receiver used in a data transmission system, this receiver utilizing this method.
2. Description of the Prior Art
It is known that in data transmission systems using carrier modulation the correct recovery of the data assumes the acquisition of several synchronizations in the receiver, more specifically the synchronization of the local carrier so as to permit coherent demodulation of the received signal, the synchronization of a local clock (alternatively denoted bit synchronization) so as to permit optimum sampling of the demodulated signal, and finally the frame synchronization (alternatively denoted word synchronization) so as to permit marking the beginning of a communication. To acquire these various synchronizations before the start of a communication, the conventional method is to have each communication be preceded by a preamble which comprises, for example, a pure carrier for the synchronization of the local carrier, an alternating sequence of "1" and "0" bits for the synchronization of the local clock and finally a known word for word synchronization. This acquisition system, which comprises phase or frequency control loops for carrier synchronization and clock synchronization, is very slow and is not so suitable for contemporary digital communication systems for data packets in which the duration of the communications can be very short, for example of the order of one second.
For these systems, which require a fast synchronization of the receiver, the initial synchronization of the receiver is preferably effected without the use of control loops, by only using a synchronizing word which is transmitted and is known in the receiver. In an article by M. H. Meyers and L. E. Franks, entitled "Joint Carrier Phase and Symbol Timing Recovery for PAM Systems", published in IEEE Transactions, Vol. COM-28, No. 8, August 1980, pages 1121-1129, a method of this type is described which is based on the calculation of the correlation function between a local synchronizing word, which is identical to the transmitted synchronizing word. The received demodulated signal is observed during a time window which is equal to the duration of the synchronizing word. In accordance with this method, local carrier synchronization and local clock synchronization are obtained simultaneously by calculating the phase values of the local carrier and the local clock which maximize the said correlation function. These optimum phase values are obtained by extrapolation on the basis of estimated values which are reasonably close to the optimum values. With this known method the determination of the local carrier phase is closely connected to the determination of the local clock phase so that it would be rather difficult to employ this method.
The present invention also permits rapid acquisition of the synchronization of the receiver by using only one synchronizing word transmitted as a preamble before a communication, but by using a totally different method as regards the synchronization of the local carrier. The invention provides a method for correcting the frequency of the local carrier, based on measuring the frequency deviations between the local carrier and the received carrier and making it possible to acquire the carrier, clock and frame synchronizations in one sampling period.